The term "diamond cut" can be very confusing, being used to mean different things at different times. Often when people refer to a cut, they are actually talking about a shape (such as round cut, cushion cut or emerald cut). Other times they may be referring to different types of diamond faceting (such as brilliant cut or stepped cut). And finally, someone may be referring to how good the proportions of a diamond are, which, as you will learn below, has a significant impact on the way light reflects and refracts in the stone and, as a result, how much it sparkles.
It is this last meaning of cut that we focus on in this guide. Choosing a well-cut stone will create a dazzling diamond that sparkles beautifully. Choose poorly, however, and the sparkle will likely disappoint, with the diamond appearing dull and underwhelming.
This guide will expand on the components of diamond cut, how diamonds are graded for cut, and the diamond cut chart used worldwide.
What is diamond cut?
The diamond cut refers primarily to the proportions of the diamond, which are measured in the following key ways:
Depth - the depth of the diamond from top to bottom as a % of the width of the diamond (60.8% in the example above).
Table - the width of the table facet (flat top of the diamond) as a % of the width of the diamond (57% in the example above).
Crown Angle - the angle between the crown facets at the top of the diamond and the girdle around the circumference of the diamond (32.0° in the example above).
Crown Height - the height of the crown (top of the diamond) as a percentage of the diameter (13.5% in the example above).
Pavilion Angle - the angle between the pavilion facets at the bottom of the diamond and the girdle. (41.2° in the example above).
Pavilion Depth - the depth of the pavilion (bottom of the diamond) as a percentage of the diameter (43.5% in the example above).
Star Length - the length of the star facets expressed as a percentage of the distance between the girdle and the edge of the table facet (50% in the example above).
Lower Half - the length of lower-half facets expressed as a percentage of the distance between the girdle and the culet (75% in the example above)
Girdle - this is the widest point of the diamond, where the crown meets the pavilion, and is given a description of its thickness as well as a percentage ('Medium to Slightly Thick' and 4.0% in the example above) and is either polished, bruted (frosted), or faceted (as in the example above)
Culet - this is the very tip at the bottom of the diamond. If it comes to a sharp point, there is no culet facet ('None' in the example above). Otherwise, the culet facet is given as a percentage of the width of the diamond.
Why is diamond cut important?
The diamond cut is what determines the "light performance" of a diamond. The trade uses terms such as brilliance, fire, scintillation and brightness to describe different elements of light performance, but for most people, the easiest way to think of it is how much a diamond sparkles!
The diamond cut determines how much light enters the diamond and how well it is reflected by the pavilion facets and refracted out of the crown facets: the more light reflected and refracted, the more intense the sparkle.
Depth (the distance from the table facet at the top of a diamond to the culet at the bottom) as a percentage of the width of the diamond is significant in determining how much a diamond will sparkle. This is because it impacts the pavilion angle (which determines how much light is reflected upwards rather than leaking out of the bottom) and the crown angle (which determines how well the diamond refracts the light coming back out of the top of the diamond).
If the diamond depth is too shallow or too deep, these angles will be sub-optimal, meaning more light will leak out of the bottom of the diamond, and less light will be reflected and refracted.
Similarly, the table (the width of the table facet as a % of the width of the diamond) is critical as a large table will let in more light but have smaller crown facets to refract the returned light. In contrast, a small table will let in less light but have larger crown facets.
With both depth and table percentages, there is some degree of personal preference, which also varies with diamond shape; however, for round brilliant cut diamonds, a depth percentage between 62.9 and 59.5% and a table percentage between 63 and 56% is generally considered to be optimal.
Diamond cut grades
The grades used for diamond cut vary slightly from lab to lab. However, the terminology developed by the Gemological Institute of America (GIA) is the most commonly used.
It is important to note that the GIA only gives round brilliant cut diamonds a cut grade. No other shapes are graded for cut, partly because they do not have consistent width (only round diamonds have a constant width - their diameter) and partly because there is less consensus about the ideal proportions for fancy cuts.
Each diamond is assessed on the quality of its cut against a set of proportions considered optimal for the light performance of a round brilliant cut diamond. Depending on how close a diamond comes to these optimal proportions, it is graded as follows:
- Excellent - the highest diamond cut grade; excellent cut diamonds will have the maximum fire and brilliance we hope for from a diamond.
- Very good - this cut is close to an excellent cut, reflecting and refracting a substantial amount of light and, therefore, having a high level of fire and life - although not as much as an excellent cut.
- Good - a diamond of this cut will still have a reasonable level of fire and life, but not as much as excellent and very good cut diamonds.
- Fair - this cut reflects some light but will not offer a high level of fire and life.
- Poor - diamonds that fall under this category will not sparkle much and are dull in appearance.
Some gemological labs, such as the International Gemological Institute (IGI), in addition to the cut grades above, also have an Ideal cut grade which sits above Excellent cut.
Related to diamond cut, there are some round brilliant cut diamonds that are marketed as Heart & Arrows diamonds due to a unique pattern created by their very precise symmetry.
What is the cheapest cut of diamond?
The lowest cut grade is Poor cut. Although diamonds with a poor cut will be cheaper, we strongly advise against them as they look dull compared to diamonds with higher cut grades.
Which diamond cut sparkles the most?
The top cut grade (Excellent cut for the GIA or Ideal cut for the IGI) will have the best light performance and therefore the most intense sparkle. However, very good cut diamonds sparkle very well and come with a more affordable price tag.
What is a GIA excellent cut?
The GIA diamond cut scale ranks diamonds from Excellent to Poor. Excellent is the highest cut grade available and is synonymous with maximum sparkle. Very good cut is second to excellent, and diamonds with a very good cut still have very good sparkle and a slightly lower price.
As a rule, at Ingle & Rhode, we only offer the top cut grades (Excellent and Very Good cut for GIA-certified diamonds and Ideal, Excellent and Very Good cut for IGI-certified diamonds).
We do this because we believe it is a mistake to compromise too much with the diamond cut grade. A larger diamond may be impressive, but if it isn't well cut, it won't have the same impact as a slightly smaller stone which sparkles more brightly. Choosing a poorly diamond cut will only disappoint. It will appear comparatively dull and lacklustre.